Unitary glass laser system with asymmetrical pumping

ABSTRACT

This specification discloses a glass laser structure comprising an integral assembly of an elongated matrix body of transparent material in which there is positioned a rod of glass laser material. An elongated flashlamp is also provided in the matrix body extending parallel to the laser rod. A section of transparent material adjacent to the side of the rod near the flashlamp has a lower index of refraction than the material of the laser rod. The remainder of the matrix has a higher index of refraction than the laser rod. Said transparent material may be made lossy for the wavelength at which the laser emits.

United States Patent [72] Inventor David W. Cull [56] Relferencm CitedR1. UNHTED STATES PATENTS 2 :5 25 3,284,722 11/1966 Gray 331/945 [45]Patented Nov. 16, 1971 Primary Examiner-Ronald L. Wibert [73] AssigneeAmerican Optical Corporation Assistant Examiner-R. J. WebsterSouthbridge, Mam. Attorneys-William C. Nealon, Noble 5. Williams andRobert J. Bird ABSTRACT: This specification discloses a glass laserstructure comprising an integral assembly of an elongated matrix body oftransparent material in which there is positioned a rod of glass lasermaterial. An elongated flashlamp is also provided [54] Wm in the matrixbody extending parallel to the laser rod. A sec- 3m in tion oftransparent material adjacent to the side of the rod 11 ants near theflashlamp has a lower index of refraction than the [52] U.S. Cl. 331/945material of the laser rod. The remainder of the matrix has a [5]] Int.Cl. l'l0ls 3/09, higher index of refraction than the laser rod. Saidtransparent H015 3/16 material may be made lossy for the wavelength atwhich the [50] Field oi Search 331/945 laser emits.

PATENTEDuuv 16 ml awe/Mm DAVID W. CUFF UNIITARY GLASS LASER SYSTEMASYMMECAL PUMPING BACKGROUND OF THE INVENTION In most laserapplications, it is usually necessary or at least desirable to limit thespread of the laser beam produced by a laser. One way to minimize thebeam spread in a glass rodtype laser is to clad the laser rod with alayer of solid transparent material which selectively absorbs at thelaser wavelength and has a higher index of refraction than the laserrod. As a result, laser emissions in the off-axial direction degeneratefollowing interaction with the cladding material. Accordingly, onlyaxial light is amplified by the laser rod and the beam spread isminimized.

Glass laser material is excited to a laser state in which it amplifieslight by illuminating the laser material periodically with ahigh-intensity flashlamp. This operation of exciting the laser is calledpumping." If a glass laser rod is pumped asymmetrically, such as by asingle flashlamp, then the side of the laser rod nearer the flashlampwill be heated to a greater degree than the side farther away from theflashlamp. An index of refraction gradient is produced from atemperature gradient in the rod generally such that the greater indexoccurs where the higher temperatures exist. Thus, in a rod which ispumped with a single flashlamp, the side of the rod nearer the flashlampwill have a greater index of refraction than the side of the rod furtheraway from the flashlamp. As a result, laser light rays travellingthrough the laser rod parallel to the axis of the rod will be benttoward the side of the rod adjacent the flashlamp. In a laser system thelaser rod is normally placed between two mirrors, which define the lasercavity. The laser light rays which travel through the rod parallel tothe axis of the rod are reflected back through the rod by the mirrors.Because these rays are bent toward the side of the rod near theflashlamp, the rays after several passes through the rod will travel tothe cladding surrounding the rod and be lost. For these reasons, theglass laser rod assemblies of the prior art employing only a singleflashlamp for pumping or some other asymmetrical pumping arrangement andutilizing a transparent cladding of selectively absorptive materialhaving a higher index of refraction than the laser rod are relativelylossy mechanisms and thus are inefficient in their operation.

SUMMARY OF THE INVENTION The laser construction of the present inventionovercomes this problem of bending of the light rays in the laser rod dueto the thermal gradient therein by employing a cladding material whichhas a slightly lower index of refraction than the laser rod on the sideof the rod adjacent to the flashlamp. The cladding material around theremainder of the rod has a slightly higher index of refraction. As aresult, light which is travelling generally parallel to the axis of therod and which is bent toward the side of the rod near the flashlamp willexperience total internal reflection when it impinges upon the interfacebetween the cladding and the rod. Upon being reflected the light willagain be bent by the gradient in the index of refraction back toparallel with the rod axis and then again toward the side of the rodnear the flashlamp to be reflected again. In this manner, the laser raysare trapped in the rod by the combined effects of bending due to theindex of refraction gradient and the total internal reflection at theinterface between the cladding and the laser rod. Since the remainder ofthe rod has cladding at a higher index of refraction, off-axis lightwill still be eliminated by either being lost immediately in thecladding or after making one bounce from the reflecting surfaceinterface between the rod and the portion of the cladding having thelower index of refraction. As a result, the desired minimum beam spreadin the laser beam produced by the system of the present invention isobtained.

Accordingly, an object of the present invention is to provide animproved glass rod laser system.

Another object of the present invention is to provide an improved glassrod laser system in which the glass rod is pumped asymmetrically.

A further object of the present invention is to improve the efficiencyof a glass rod laser system which is asymmetrically pumped.

A still further object of the present invention is to overcome theproblem of bending of the light rays in a glass rod laser due to thethermal gradient in the glass rod.

Further objects and advantages of the present invention will becomereadily apparent as the following detailed description of the inventionunfolds and when talten in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of oneend portion of the laser structure of the present invention;

FIG. 2 is a longitudinal sectional view taken along the lines 2-2 ofFlG.l; and,

FIG. 3 is a cross-sectional view taken substantially at the lines 3-3 ofFIG. 2 and looking in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in the drawing, thelaser system of the present invention comprises a laser rod ll made ofglass laser material. The laser rod 111 for example may be made of glassdoped with neodymium. The rod 11 is mounted in an elongated glass matrix13 which extends the length of the rod 11. The matrix is oval in crosssection and the rod 11 is located on one side of the matrix. The matrix13 is made of material which is lossy to light at the wavelength of thelaser beam produced by the system but which will transmit light forpumping the laser material of the rod 11. The matrix material may forexample by samarium-doped glass. Also, within the matrix 13 on theopposite side thereof from the glass rod 111 is an elongated flashlamp15 for pumping the rod to excite the material thereof to a laser state.Contiguous to the rod ll on the side adjacent the flashlamp 15, there isprovided a sectionallike layer of transparent material 19 whicharbitrarily can be lossy to the laser light wavelength and is lighttransmissive to the pump light.

The section 19 may be considered to be part of the matrix 13 and mayalso be samarium-doped glass. The section 19 is selected to have anindex of refraction lower than that of the rod 11 while the remainder ofthe glass matrix 113 is selected to have a higher index of refractionthan that of the glass rod 11. The difierent indices of refraction inthe section 19 and in the remainder of the matrix 13 are achieved bydifferent levels of doping. The section l9 as illustrated in FIG. 2extends throughout the length of the rod, and as illustrated in FIG. 3is crescent shaped in cross section. The assembly of the rod ll, thesection 19 and the remainder of the matrix are fused together to form aunitary composite structure. The outside wall of the matrix 113 iscovered by a silver reflector 20 to reflect the pump light produced bythe flashlamp 15 within the matrix. At opposite ends of the rod II aremirrors 21 and 23 defining the laser cavity of the system. The flashlampl5 comprises a cylindrical bore through the matrix 13 extending parallelwith the rod Ill. The ends of the bore are closed and the bore is filledwith an inert gas such as xenon or ltrypton. Electrode terminals 16 and17 are mounted in opposite ends of the bore for passing an electricdischarge through the inert gas to produce a high-intensity flash oflight for pumping the laser rod 11. The material of the matrix istransmissive to the pump light produced by the flashlamp 1:5.

The above-described structure can be manufactured by cladding a rod oflaser material with an annulus of transparent material having a lowerindex of refraction than the laser rod. A major portion of the claddingis then removed by cylindrical finishing on eccentrically positionedaxis so that only the section 19 remains on one side of the rod. Theresulting com posite cylinder is then inserted into a cylindrical boreof a preshaped matrix body of higher index of refraction than thematerial of the laser rod and the assembly is fused together.

Pump light produced by the flashlamp 15 will illuminate the laser rod111 and excite it to a laser state. When the laser material of the rodis excited to a laser state, it will emit light of a particularwavelength, which for neodymium-doped glass is normally 1.06 microns.When light of this wavelength travels through the laser material of therod, it will stimulate further emission of light of this wavelength andthe stimulated emission will be in phase with the stimulating light. Asa result, light travelling through the rod will experienceamplification. Emissions which are not travelling parallel with the axisof the rod will enter the selectively lossy material surrounding the rodand degenerate. However, those which travel parallel with the axis willbe amplified throughout the length of the rod 11 and be reflected by themirrors 21 and 23 for further amplification in the rod. As a result,standing waves of light energy called laser oscillations are set upwithin the rod 1 l. The mirror 23 is made partially transmissive so thatsome of the light energy from the laser oscillations are transmitted tothe target.

When the flashlamp l5 pumps the rod 11 it also introduces heat into therod 11. Because of the asymmetrical pumping arrangement inherent in theuse of a single flashlamp, the side of the rod 11 nearer the flashlamp15 will be heated to a greater degree than the side of the rod 11further away from the flashlamp l5, and a temperature gradient willexist across the rod 11. Because of this temperature gradient, the sideof the rod 11 nearer the flashlamp 15 will having a higher index ofrefraction than the side of the rod 11 further away from the flashlamp.As a result, a light ray travelling generally parallel to the axis ofthe rod 11 will be gradually bent toward the side thereof having thegreater index of refraction or, in other words, will be bent toward thecladding section 19. Then the light ray impinges upon the interface 27between the rod 11 and the cladding section 19, it will have a low angleof incidence. Because the section 19 has a lower index of refractionthan that of the rod 11, the light ray will experience total internalreflection at the interface and, accordingly, will be reflected back upinto the rod. The light ray then will again be bent parallel with theaxis of the rod and finally bent again toward the section 19 of lowerrefractive index material where the ray will again experience totalinternal reflection. This bending of the light by the thermal gradientis gradual and continuous between successive reflections. Accordingly, alight ray will make numerous passes through the rod between reflectionsat the interface between the rod 11 and the section 19. in this manner,the laser light is trapped within the rod 11 by the combined effects ofthe gradient in the index of refraction and the total internalreflection at the interface between the rod 11 and the section 19 oflower index material. The dashed line illustrates the path of a typicallight ray travelling generally parallel to the axis of the rod 11. inthis manner, the effect of the thermal gradient in the rod 11 isovercome and a highly efficient asymmetrically pumped laser rod systemis provided. Moreover, the system will produce a beam having a minimumbeam spread since off-axial light will still enter the cladding eitherdirectly or after one bounce from the interface between the rod 11 andthe section 19.

The above description is of a preferred embodiment of the invention andmany modifications may be made thereto without departing from the spiritand scope of the invention, which is defined in the appended claims.

Having described my invention, 1 claim:

1. A laser system including pumping means and laserable materialcomprising a rod of laser material, transparent selectively absorbingmaterial surrounding said rod, the transparent selectively absorbingmaterial adjacent to said rod on the one side thereof which is adjacentsaid pumping means having a lower index of refraction than that of saidrod, the remaining transparent selectively absorbing material adjacentto said rod having an index of refraction at least as high as that ofsaid rod.

2. A laser system as recited in claim 1 wherein said laser material isglass.

3. A laser system as recited in claim 1 wherein said remainingtransparent selectively absorbing material has an index of refractionhigher than the index of refraction of said rod.

4. A laser system as recited in claim 1 further comprising means to pumpsaid rod asymmetrically.

5. A laser system as recited in claim 4 wherein said means to pump saidrod comprises an elongated flashlamp extending parallel to said rod.

6. A laser system as recited in claim 5 wherein said transparentselectively absorbing material comprises a matrix enclosing both saidflashlamp and said rod.

7. A laser system as recited in claim 6 wherein said matrix is coveredwith a reflector to reflect light within said matrix produced by saidflashlamp.

8. A laser system comprising a rod of laser material, an elongatedflashlamp extending parallel to said rod for pumping said rod, alight-transmissive matrix of selectively absorbing material surroundingsaid flashlamp and said laser rod, a section of said matrix adjacent tothe side of said rod near said flashlamp having an index of refractionlower than that of said rod, the remainder of said matrix having anindex of refraction at least as high as that of said rod.

9. A laser system as recited in claim 8 wherein said laser material isglass.

10. A laser system as recited in claim 8 wherein said remainder of saidmatrix has an index of refraction higher than said rod.

11. A laser system as recited in claim 8 wherein said matrix is coveredwith a reflecting layer to reflect light from said flashlamp back intosaid matrix and to said rod.

1. A laser system including pumping means and laserable materialcomprising a rod of laser material, transparent selectively absorbingmaterial surrounding said rod, the transparent selectively absorbingmaterial adjacent to said rod on the one side thereof which is adjacentsaid pumping means having a lower index of refraction than that of saidrod, the remaining transparent selectively absorbing material adjacentto said rod having an index of refraction at least as high as that ofsaid rod.
 2. A laser system as recited in claim 1 wherein said lasermaterial is glass.
 3. A laser system as recited in claim 1 wherein saidremaining transparent selectively absorbing material has an index ofrefraction higher than the index of refraction of said rod.
 4. A lasersystem as recited in claim 1 further comprising means to pump said rodasymmetrically.
 5. A laser system as recited in claim 4 wherein saidmeans to pump said rod comprises an elongated flashlamp extendingparallel to said rod.
 6. A laser system as recited in claim 5 whereinsaid transparent selectively absorbing material comprises a matrixenclosing both said flashlamp and said rod.
 7. A laser system as recitedin claim 6 wherein said matrix is covered with a reflector to reflectlight within said matrix produced by said flashlamp.
 8. A laser systemcomprising a rod of laser material, an elongated flashlamp extendingparallel to said rod for pumping said rod, a light-transmissive matrixof selectively absorbing material surrounding said flashlamp and saidlaser rod, a section of said matrix adjacent to the side of said rodnear said flashlamp having an index of refraction lower than that ofsaid rod, the remainder of said matrix having an index of refraction atleast as high as that of said rod.
 9. A laser system as recited in claim8 wherein said laser material is glass.
 10. A laser system as recited inclaim 8 wherein said remainder of said matrix has an index of refractionhigher than said rod.
 11. A laser system as recited in claim 8 whereinsaid matrix is covered with a reflecting layer to reflect light fromsaid flashlamp back into said matrix and to said rod.